1. Field of the Invention
This invention relates to an improvement in the production of TiCl.sub.3 -containing compositions for use as a catalyst component in olefin polymerization. More particularly, it relates to the production of TiCl.sub.3 compositions which, when combined with an appropriate alkyl compound, result in highly stereospecific catalysts for propylene polymerization.
2. Description of the Prior Art
Polymers and copolymers of 1-olefins have achieved prominence in recent years since the discovery of the so-called transition metal or coordination catalysts, which make possible the production of such polymers, having very desirable properties, at relatively mild conditions of temperature and pressure. Of particular interest are the highly crystalline polymers of propylene, which have become known as stereoregular, or isotactic or stereosymmetric polypropylene, and also highly crystalline copolymers, e.g., block copolymers, and certain random copolymers that are formed by polymerizing propylene with a small amount of another 1-olefin, e.g., ethylene.
It is well known to polymerize olefins to linear polymers and, specifically, to polymerize propylene to stereoregular polypropylene by contact with catalysts having a titanium trichloride-containing component and a dialkylaluminum monohalide component.
It is known to produce the titanium trichloride catalyst components for such polymerization catalysts by reacting titanium tetrachloride in solution with an aluminum alkyl compound as reducing agent and to convert the product of the reduction by heating to one in which the titanium trichloride is present in a catalytically highly active and highly stereo-regulating form.
Polypropylene produced with certain such stereoregulating catalysts is generally referred to as "isotactic" polypropylene. This is characterized by the presence of propylene units in each molecule in sterically identical arrangement. The "isotactic" polypropylene component of sufficiently high molecular weight is insoluble in boiling n-heptane and in boiling diethyl ether and is also insoluble in C.sub.5 -C.sub.8 aliphatic hydrocarbons at room temperature.
One of the desirable properties of commercial polypropylene is a high yield stress value, preferably at least 33 MN/m.sup.2.
While it is desirable for commercial purposes to produce polypropylene of high "isotacticity", it is also desirable to control the polymerization so as to produce a high yield of polypropylene per unit weight of catalyst; for example, if polypropylene can be produced at a yield of more than 3000 grams per gram of TiCl.sub.3, the product may be commercially useful without the conventional treatment for catalyst residue removal. Even if such extremely high activity is not obtained, the higher activity of the catalyst results in substantial process economies. For purposes of this specification, catalyst activity is measured in units of grams of polymer per gram of TiCl.sub.3 per hour per bar of pressure.
A number of catalyst systems have been described which are capable of producing polypropylene of desirable commercial properties in relatively high yield. Such catalysts systems, which are further designed to produce polymer of high bulk density, are described, for example, in U.S. Pat. No. 3,562,239 to DeJong et al and in co-assigned U.S. patent application Ser. No. 316,191, filed Dec. 18, 1972 of Van Der Bend et al now U.S. Pat. No. 3,857,795. The TiCl.sub.3 component of the catalyst of U.S. Pat. No. 3,562,239, as claimed, is produced by adding titanium tetrachloride to a trialkylaluminum solution at temperatures below -30.degree.C but not below -90.degree.C; thereafter gradually warming the mixture and maintaining it below 80.degree.C until the reaction is essentially complete; and thereafter heating the mixture above 80.degree.C, e.g., between 100.degree. and 200.degree.C, until the titanium trichloride therein is converted to violet gamma titanium trichloride.
According to said U.S. Pat. No. 3,857,795, the TiCl.sub.3 component is produced by adding titanium tetrachloride to a trialkylaluminum solution at temperatures below -90.degree.C, at certain controlled addition rates, followed by gradual controlled warming of the reaction mixture. This permits production, at will, of dense, smooth surfaced catalyst particles which result in a non-dusting high bulk density polymer, or of catalyst particles having a relatively porous surface which are particularly adapted to the production of block copolymers of relatively high bulk density.
In the polymerization of propylene, the titanium trichloride components produced by reduction of TiCl.sub.4 are combined with an aluminum alkyl cocatalyst, preferably aluminum diethyl chloride.
In the patent art and literature of the polymerization on propylene via titanium trichloride-aluminum alkyl based catalysts, numerous methods and expedients have been described for modifying catalyst preparation conditions, catalyst compositions or polymerization conditions in order to achieve various desirable purposes such as producing catalysts having certain characteristics, polymers having certain characteristics or certain process advantages.
In the reaction of TiCl.sub.4 with an alkyl aluminum compound, aluminum chlorides, such as aluminum ethyl dichloride and AlCl.sub.3, are produced besides TiCl.sub.3. The desired product is TiCl.sub.3, but, in order for the TiCl.sub.3 to be useful in producing highly crystalline polymers having a good yield stress (strength) in good yields, it has been suggested to remove the aluminum chlorides from the TiCl.sub.3. It appears that the removal of AlCl.sub.3 is quite difficult since AlCl.sub.3 and TiCl.sub.3 are isomorphous and crystallize readily together in the same crystalline lattice. Powerful complexing agents such as diphenyl ether and dibutylether have been suggested for said removal of AlCl.sub.3. The disadvantage of the use of such agents is that they may have a deleterious effect on the yield stress and consequently will have to be carefully washed out of the TiCl.sub.3 if used in excess. If so, it will be quite difficult, moreover, to completely remove the agent, e.g. ether. It has been found that the use of excessive amounts of complexing agent may result in permanent damage to the TiCl.sub.3, not only resulting in loss of yield stress but also in diminished yield.
One expedient that has been described in the literature to modify a TiCl.sub.3 catalyst component is washing with a suitable solvent prior to combining it with aluminum alkyl for use in the polymerization reaction. The following are representative references of this type:
United Kingdom Pat. No. 948,447, published Feb. 5, 1964, discloses a process for production of a titanium trichloride component for a polymerization catalyst by reducing a transition metal compound such as titanium tetrachloride by reaction with an alkyl aluminum halide in an inert liquid medium at a temperature from -100.degree.C to 0.degree.C, preferably between -21.degree.C and -70.degree.C, and thoroughly washing the resultant precipitate before use in the polymerization. Repeated thorough washing is disclosed to be essential. Liquids disclosed for use in the reduction reaction are propylene trimer, hexane, and hydrogenated oxygen-free diesel oil. Washing is preferably carried out "with an oxygen-free and hydrogenated hydrocarbon". Washing at an elevated temperature is not disclosed.
United Kingdom Pat. No. 960,232, to the same patentee as the above, is directed to a process for the production of olefin polymer or copolymer of high bulk density by polymerizing the olefins in the presence of a liquid dispersing agent, diethyl aluminum monochloride and a titanium-containing catalyst component which has been prepared from titanium tetrachloride and diethyl aluminum monochloride or aluminum ethyl sesquichloride at a temperature within the range of 0.degree. to 20.degree.C at specified reactant concentrations and mole ratios. The precipitated titanium trichloride-containing catalyst component is advantageously separated and washed out with an inert solvent or alternatively, the catalyst component suspension, before separating it from the dispersing agent, is subjected to an additional thermal treatment at temperatures between 40.degree. and 150.degree.C and then washed out. Example 2 of the patent shows a preparation in which the suspension was heated at 90.degree.C but then cooled before the washing step. There is no disclosure of washing at an elevated temperature.
U.S. Pat. No. 3,058,970, to Rust et al, discloses a process for preparing a TiCl.sub.3 catalyst component by reducing TiCl.sub.4 with a dialkylaluminum halide between -20.degree. and +15.degree.C, followed by "annealing" between 60.degree. and 100.degree.C. It is disclosed that the annealing may be brought about by hot washing, for example, in "toluene, cyclohexane, methylcyclohexane, heptane, isooctane, or hydrogenized diesel oils boiling between 180.degree. and 280.degree.C". Hot washing is not further defined in the specification; in each of the illustrative examples the reaction mixture is cooled after the annealing step and is not separated from the liquid hydrocarbon at an elevated temperature.
Hot washing with a mixed solvent in which one component may be a hydrocarbon and the other is selected from a group of nonhydrocarbon compounds is disclosed is U.S. Pat. No. 3,825,524 to Wada et al, in which the "extraction" can be conducted at a temperature ranging from room temperature to the boiling point of the main solvent.
Another related disclosure is that of U.S. Pat. No. 3,640,987 to Phung et al, which is directed to a two-stage reduction of TiCl.sub.4 by means of a hydride or organometallic compound, in which the first stage is carried out in low boiling solvent at temperatures below 50.degree.C and the reduction is completed in a high boiling solvent at a temperature higher than 75.degree.C.